For glMapBuffer only, specifies the access policy, indicating whether it will be possible to read from, write to, or both read from and write to the buffer object's mapped data store. The symbolic constant must be GL_READ_ONLY, GL_WRITE_ONLY, or GL_READ_WRITE.

glMapBuffer maps to the client's address space the entire data store of the buffer object currently bound to target. The data can then be directly read and/or written relative to the returned pointer, depending on the specified access policy. If the GL is unable to map the buffer object's data store, glMapBuffer generates an error and returns NULL. This may occur for system-specific reasons, such as low virtual memory availability. If no error occurs, the returned pointer will have an alignment of at least GL_MIN_MAP_BUFFER_ALIGNMENT basic machine units. The value of GL_MIN_MAP_BUFFER_ALIGNMENT can be retrieved by calling glGet() with pname set to GL_MIN_MAP_BUFFER_ALIGNMENT and must be a power of two that is at least 64.

If a mapped data store is accessed in a way inconsistent with the specified access policy, no error is generated, but performance may be negatively impacted and system errors, including program termination, may result. Unlike the usage parameter of glBufferData, access is not a hint, and does in fact constrain the usage of the mapped data store on some GL implementations. In order to achieve the highest performance available, a buffer object's data store should be used in ways consistent with both its specified usage and access parameters.

A mapped data store must be unmapped with glUnmapBuffer before its buffer object is used. Otherwise an error will be generated by any GL command that attempts to dereference the buffer object's data store. When a data store is unmapped, the pointer to its data store becomes invalid. glUnmapBuffer returns GL_TRUE unless the data store contents have become corrupt during the time the data store was mapped. This can occur for system-specific reasons that affect the availability of graphics memory, such as screen mode changes. In such situations, GL_FALSE is returned and the data store contents are undefined. An application must detect this rare condition and reinitialize the data store.

A buffer object's mapped data store is automatically unmapped when the buffer object is deleted or its data store is recreated with glBufferData.

GL_INVALID_ENUM is generated if target is not one of the accepted targets.

GL_INVALID_ENUM is generated if access is not GL_READ_ONLY, GL_WRITE_ONLY, or GL_READ_WRITE.

GL_OUT_OF_MEMORY is generated when glMapBuffer is executed if the GL is unable to map the buffer object's data store. This may occur for a variety of system-specific reasons, such as the absence of sufficient remaining virtual memory.

GL_INVALID_OPERATION is generated if the reserved buffer object name 0 is bound to target.

GL_INVALID_OPERATION is generated if glMapBuffer is executed for a buffer object whose data store is already mapped.

GL_INVALID_OPERATION is generated if glUnmapBuffer is executed for a buffer object whose data store is not currently mapped.